1. Field of the Invention
The present invention relates to a radio communication system including a plurality of base stations and particularly to a method for deciding tilt angles used in deciding a tilt angle of an antenna located in each base station to reduce a deterioration rate of the entire radio communication system, and to an apparatus for deciding the same.
2. Description of the Related Art
Concerning a radio communication system including a plurality of base stations that provide radio communication lines to fixed users and mobile users distributed over a plurality of areas, when constructing such radio communication system or locating an additional radio station in an existing radio communication system, a tilt angle of an antenna having directivity in a vertical plane and located in each base station is decided so that the radio communication system can provide high-quality radio communication lines.
Tilt angles are not decided for all antennas arranged in respective base stations but only for antennas which are considered to be effective for the radio communication system to provide higher-quality radio communication lines if tilt angles thereof are changed from predetermined initial angles. Such antennas are selected and tilt angles of the selected antennas are decided by a person in charge in the following procedures.
First of all, a propagation loss from each antenna to a predetermined point is obtained by a propagation simulator based on information on the location, altitude, building, geography, etc. of the base station. Thereafter, reception power when a signal transmitted from each antenna is received at the predetermined point is calculated based on transmission power from the antenna, orientation of the antenna, a beam pattern of the antenna in horizontal and vertical planes, and the propagation loss obtained earlier. In addition, reception quality represented by S/N (signal-to-noise) ratio or SIR (signal power to interference ratio) is calculated.
Here, the point in which a predetermined value of reception power or reception quality is not satisfied is defined as a “deterioration point.”
From the results of the above calculations, a deterioration rate of coverage of each antenna is calculated. Coverage of each antenna is defined as an area where reception power of a signal from each antenna is larger than reception power of a signal from any other antenna and, at the-same time, is a predetermined value or larger. An area to be covered by the radio communication system is divided and allocated to respective radio stations which constitute the radio communication system. Each of the allocated areas serves as the coverage, and the coverage of the whole system is equal to the sum of the coverage of the respective antennas. A deterioration rate is defined as a ratio of the deterioration points within specified coverage.
With the definitions stated above, a deterioration rate of each antenna and a deterioration rate of the entire system are obtained. A display device provided in the propagation simulator displays coverage of each antenna so that deterioration points in the coverage can be recognized. A person in charge of deciding tilt angles of antennas selects antennas whose tilt angles are to be changed based on the displayed content and designates the tilt angles.
FIG. 5 is a view showing adjusting operations for tilt angles of antennas. The tilt angles of antennas 1 and 2, each having a beam pattern, are adjusted to improve deterioration rates (deterioration rates of coverage of respective antennas) X % and Y %, which represent communication qualities within the coverage of the antennas, as well as a deterioration rate of the entire system Z %.
By changing a large tilt angle of an antenna to a smaller angle, in other words, by changing a situation where transmission power is concentrated to an inner side (more front side) than original coverage of the antenna to a situation where transmission power is distributed within the original coverage of the antenna, reception power within the coverage of the antenna tend to increase.
On the contrary, by changing a small tilt angle of an antenna to a larger one, in other words, by changing a situation where sufficient transmission power is distributed within original coverage of the antenna to a situation where transmission power is concentrated to an inner side (more front side) than the original coverage of the antenna, reception power within the coverage of the antenna tends to decrease. As a result, the deterioration rate of the antenna tends to increase. However, in coverage with a sufficiently small deterioration rate, an increase in deterioration rate due to an increase in tilt angle is often very small. In such a case, power which interferes with the coverage of neighboring antennas is reduced, and therefore, in many cases, the deterioration rates of coverage of the neighboring antennas are reduced.
Tilt angles have been decided by a person in charge based on the above described general trends to reduce deterioration rates.
With regard to adjustment of a tilt angle of an antenna, Patent Document 1 (Unexamined Japanese Utility Model Publication No. Hei02-135884) disclosed a calculator which, while searching a target by using an antenna of radar, calculates a tilt angle using a search distance and an installation height of the antenna, and a comparison controller which compares an output from the calculator to a tilt angle signal outputted from the antenna to control the tilt angle.
Patent Document 2 (Unexamined Japanese Patent Publication No. 2002-095040) disclosed that, when designing and adjusting or operating a radio network, operation parameters are decided by using an optimization process so that design and adjustment of a radio network can be done more easily. This Document suggests orientation of an antenna as one of the operation parameters.
Patent Document 3 (Unexamined Japanese Patent Publication No. Hei11-166964) has disclosed a method used when correcting an angle of an antenna with a step tracking system in order to conduct satellite tracking. In this method, an angle of an antenna is corrected by a large step size in the initial correction stage, and the step size is thereafter gradually reduced so that the antenna is oriented in an optimal direction.
Of these techniques stated above, the one described in Patent Document 1 relates to a radar, and there is no description about tilt angle control for antennas with consideration of a deterioration rate of the entire radio communication system having a plurality of antennas.
As for the technique disclosed in Patent Document 2, it is stated that operation parameters including orientation of an antenna are decided by using an optimization process when designing, adjusting and operating a radio network. However, a specific method described therein was only the use of a network optimization process and therefore the content of the method is not clear.
The technique disclosed in Patent Document 3 is for correction of an angle of an antenna with step track system for conducting satellite tracking. Similarly to Patent Document 1, there is no description of tilt angle control for an antenna with consideration of a deterioration rate of the entire radio communication system.
Patent Document 1: Unexamined Japanese Utility Model Publication No. Hei02-135884
Patent Document 2: Unexamined Japanese Patent Publication No. 2002-095040
Patent Document 3: Unexamined Japanese Patent Publication NO. Heisei 11-166964
As described earlier, a person in charge selects antennas for changing tilt angels in a radio communication system and decides the tilt angles thereof with consideration of a propagation environment. However, the results vary depending on experience of the person in charge, and a deterioration rate of the entire system therefore becomes inconsistent. Accordingly, a stable quality cannot be maintained. Moreover, selecting antennas from many of them and deciding their angles have been time-consuming operations.
Furthermore, when deciding a tilt angle, a tilt angle of the same antenna is sometimes changed more than once and propagation simulation is carried out each time the tilt angle is changed. At this time, if an angle is changed largely, the angle may exceed an optimal value and be far from it, whereas a small degree of angle change requires a number of angle changes. Therefore, it is important to set a degree for an angle change carried out in adjusting an angle.
It has been very difficult to obtain tilt angles which enable sufficiently small deterioration rate of the entire system within a limited period of time particularly in a large radio communication system where an enormous number of antennas are located.
Moreover, where an antenna with a certain tilt angle interferes with a neighboring area, affecting optimal tilt angles of other antennas, in other words, in the case of a radio communication system where antennas are affecting each other (for example, CDMA (Code Division Multiple Access)), optimization of tilt angles has been even more difficult. Since antennas affect each other, if one of the antennas is adjusted, the adjustment result affects other antenna, requiring further adjustment of the affected antenna. Further, adjustment of an antenna does not always affect the neighboring area only, making the optimization even more difficult. Therefore, it has been very difficult to obtain tilt angles which enable a sufficiently small deterioration rate of the entire system.